1. Field of the Invention
The present invention relates to a composite mobile communication device operable in different mobile communications system such as both PHS (personal handy phone system) and PDC (personal digital cellular) system, and more specifically, it relates to a local circuit of the mobile communication device.
2. Description of the Related Art
In a mobile communication terminal such as a digital mobile phone, a local circuit including an oscillator and a phase-locked loop (PLL) circuit is used particularly for frequency conversion at the time of transmission and reception. In general, the mobile communication terminal is provided with a single local circuit for a radio-frequency (RF) stage.
With the progress of compound of movable terminals, composite terminals compatible with either of PHS and PDC have been used for utilizing the advantages of both of them.
For example, Japanese Patent Laid Open Application (JP-A) No. 9-163450 discloses an example of such a composite terminal. More specifically, the conventional composite mobile terminal for both PDC and PHS is composed of a dual-mode receiver, a dual-mode transmitter, and a controller. The dual-mode receiver includes a RF local oscillation circuit for supplying two local oscillation signals for PHS and PDC. The dual-mode transmitter includes a local oscillation circuit for PHS and another local oscillation circuit for PDC. Such a configuration is capable of realizing both functions of the cordless type radio phone and the cellular type radio phone.
With the recent spread of mobile computing, the demand for high-speed data communication grows more and more. To achieve high-speed data communication in a PHS system, for example, a 64 Kbps communication rate of ISDN (integrated services digital network), two-slot assignment for each of transmission and reception timings in a frame is employed on time-division duplex (hereinafter, called xe2x80x9cone-frame-two-slot T/R assignmentxe2x80x9d). More specifically, two transmission slots and two reception slots for a TDMA/TDD frame are used for high-speed data communication. In multi-carrier TDMA/TDD system such as the PHS system, therefore, the RF stage needs two local oscillators which can generate different local frequencies corresponding to two slots of the one-frame-two-slot T/R assignment, respectively.
In a PHS/PDC-composite mobile terminal which can provide the one-frame-two-slot T/R assignment, it is considered that a switch is provided in the PHS RF stage to select one of the two local oscillators for two slots of the one-frame-two-slot T/R assignment depending on which one of the two slots the current timing is.
Since two PHS local circuits are prepared for executing the one-frame-two-slot T/R assignment, however, the circuit configuration of the PHS transceiver becomes complicated and the amount of hardware is increased. The increased size and weight should be avoided.
An object of the present invention is to provide a composite mobile communication device which allows the high-speed communication without increasing in size and weight.
Another object of the present invention is to provide a composite mobile communication device which can obtain the high-speed communication according to the one-frame-two-slot T/R assignment on TDD scheme with reduced amount of hardware.
According to the present invention, a mobile communication device is selectively operable in first and second digital communication schemes. The mobile communication device includes; a first local circuit for generating a first local oscillation signal to supply it to a first communication circuit for the first digital communication scheme; and a second local circuit for generating a second local oscillation signal to supply it to a second communication circuit for the second digital communication scheme. The mobile communication device further includes a switching circuit for switching the first local oscillation signal between a first frequency and a second frequency. One of the first frequency and the second frequency is produced from the second local oscillation signal.
The first digital communication scheme preferably includes a time-division duplex scheme allowing a one-frame-two-slot assignment communication by which the first frequency and the second frequency are assigned to two different time slots for each of transmission and reception, respectively.
Further preferably, the first digital communication scheme is a digital cordless phone scheme and the second digital communication scheme is a digital cellular phone scheme.
According to another aspect of the present invention, a first local oscillation signal is generated which is used for a first communication circuit for the first digital communication scheme and a second local oscillation signal is generated which is used for a first communication circuit for the first digital communication scheme. A third local oscillation signal is produced from the second local oscillation signal. When the first communication circuit operates according to a one-frame-two-slot assignment communication on time-division duplex scheme, the first local oscillation signal and the third local oscillation signal are selectively supplies to the first communication circuit depending on two different time slot timings of the one-frame-two-slot assignment.
Preferably, when the second communication circuit is in a reception-OFF state in an intermittently receiving mode, the first local oscillation signal and the third local oscillation signal are selectively supplied to the first communication circuit. When the second communication circuit is in a reception-ON state in the intermittently receiving mode, the first local oscillation signal to supplied to the first communication circuit according to a one-frame-one-slot assignment communication on the time-division duplex scheme.